This invention is directed to negative-acting photoimageable compositions which are developable in alkaline aqueous solutions. The invention is particularly applicable to primary photoimaging resists, but is applicable, as well, to compositions that are hardenable so as to form solder masks and the like.
A variety of such photoimageable compositions are described throughout the patent literature. Essential components of the type of photoimageable compositions to which the present invention is directed include I) a binder polymer, II) photopolymerizable .alpha.,.beta.-ethylenically unsaturated compound(s), and III) a photoinitiator chemical system. The binder polymer I) has sufficient acid functionality, generally carboxylic acid functionality, that the binder polymer is soluble in dilute alkaline aqueous solution and thereby renders the photoimageable composition developable in such alkaline aqueous solutions. The photopolymerizable compound(s) II) are monomers and/or short chain oligomers, a substantial portion of which have multiple .alpha.,.beta.-ethylenic unsaturated functionality.
The photoinitiator chemical system III) includes chemicals which generate free radicals upon exposure to actinic radiation. These free radicals propagate the polymerization of the .alpha.,.beta.-ethylenic unsaturated moieties of the photopolymerizable compounds II). Herein, the photoinitiator system II) is deemed to include not only chemical compounds which generate free radicals, but catalysts or sensitizers which promote the free-radical initiated polymerization of the .alpha.,.beta.-ethylenic unsaturated moieties of the photopolymerizable compounds II).
Printed circuit boards almost invariably have through-holes to establish connections with circuitry on opposite faces of the board. Photoresists are required to "tent" these through-holes during processing. With holes becoming larger on circuit boards, higher tenting strength is becoming increasingly important; thus greater flexibility of photoimageable compositions after development is required. Improved flexibility also contributes to improved cross hatch adhesion which allows for better compatibility with automated polyester support film removal systems used to separate a support film from the photoresist after exposure and before development. If the photoresist is brittle, these support film removal systems will cause chipping of the exposed areas of photoresist predominantly at the edges of the panel and subsequently, circuit line defects.
By replacing a portion of conventional photoreactive monomers (like ethoxylated trimethylolpropane triacrylate) with an isocyanuric, urethane-based oligomer, a significant improvement to tenting strength and flexibility was observed. However, even though the flexibility was noticeably better, the fine line adhesion was not improved and the oligomer was shown to be a major source of developer scumming.
Improved flexibility, fine line adhesion and lower developer scumming has been demonstrated when the isocyanuric, urethane-based oligomer is comprised of the product of a polyethoxymono(meth)acrylate and the isocyanurate trimer of hexamethylene diisocyanate, as described, for example, in U.S. Pat. No. 5,744,282. The use of a (meth)acrylate-functional urethane product formed from a mono- or polyalkoxymono(meth)acrylolyl ester, such that the (meth)acrylate functionality is separated from the urethane linkage by one or more flexible alkylene oxide groups, in UV-curable photoresists enhances the performance of such compositions over those made with urethane compounds based on the isocyanurate trimer of hexamethylene diisocyanate. Alternatively, urethane oligomers have been proposed that are formed from monoalkoxymono- or di-caprolactone(meth)acrylolyl esters, which adds a mono- or di-caprolactone chain extension between the monoalkoxy(meth)acrylate functionality and the urethane linkage. Present day commercial applications require further improvements to flexibility, fine line adhesion and developer scumming, while not interfering with the chemical resistance of the photoresist to processing solutions and its stripping ability after formation of the patterned copper circuit lines.
Herein, novel (meth)acrylate-functional urethane oligomers based on polyalkoxy/polylactone (meth)acrylolyl esters are incorporated as at least a portion of the photopolymerizable component II). The (meth)acrylate-functional urethane oligomers of this invention are found to significantly improve flexibility and fine line adhesion of the photoresist and minimize developer scumming. Along with improving the aforesaid properties, it has been found that (meth)acrylate-functional urethane oligomers further enhance the chemical resistance of the exposed photoresist to processing solutions, such as developing, plating and etching solutions, and also its stripping ability in strong alkaline aqueous solutions.
Certain current circuit board manufacturing processes require further improved stripping ability, and improved resolution is always a goal in improved photoresists. In one common manufacturing process, the process begins with a circuit board blank comprising a dielectric substrate, such as fiberglass-reinforced epoxy resin, having a copper layer on each surface. In an initial step through or via holes are drilled. A catalyst is applied to the interior surfaces of these holes allowing a thin layer of copper to be deposited through the holes by electroless plating. Then the entire board is covered on both sides with photoresist, which photoresist is exposed and developed. The board is then electroplated with copper to add sufficient copper in the through holes for adequate electrical conductivity, typically to a thickness of at least one mil (25 microns), and to build up circuitry traces in the exposed regions of the copper surface. Next, a tin or tin/lead metal is plated on exposed copper to act as an alkaline etch resist after stripping of the photoresists. The resist is stripped with an alkaline stripping solution, such as a 2-3% sodium hydroxide solution. Then, the board is alkaline etched, e.g., with ammonium hydroxide solution of about pH 10 or above, to remove copper from between the circuitry traces.
The current trend is for boards to become thicker and via holes to become smaller. In order that sufficient copper be electroplated in the via holes, plating times are increased such that the build-up of circuitry traces often excedes the height of the photoresist pattern, sometimes by as much as 2 mils (50 microns), resulting in mushrooming of the electroplated copper over the photoresist pattern. The overhang of this mushrooming tends to create problems with stripping. While the overhang could be addressed with thicker photoresist layers, resolution would be compromised. Accordingly, it is desirable that a photoresist strip cleanly despite the overhang problem. If a photoresist strips in large particle sizes, it tends to become trapped beneath the overhang, inhibiting etching, causing circuit defects and scrap panel.